FIG. 1 recalls the main elements making up a power assisted steering system, for example electric, of an automobile. Such a steering system comprises, on one hand, a mechanical portion comprising a steering wheel 2 rotatably connected to a steering column 3, whereof the remote end of the wheel 2 carries a steering pinion engaged with a steering rack 4, slidingly mounted in a steering gear-box 5. The two opposite ends of the steering rack 4 are connected, respectively, via connecting rods 6 and 7, to the right and left steering wheels (not shown) of the vehicle. The steering system comprises, to assist the manual force exerted by the vehicle's driver on the wheel 2, an electric assist motor 8 with two directions of rotation. The output shaft of the assist motor is coupled, via a speed reducer 9, in particular with worm and cylindrical worm wheel or worm train, to the steering column 3 or the steering pinion or the steering rack 4 directly, so as to transmit an engine torque (possibly also a resistant torque) to said element. The electric assist motor 8 is controlled by an onboard electronic computer 10, which receives and processes various signals, coming from sensors.
In one usual embodiment, the electronic computer 10 in particular receives an electric signal coming from a torque sensor 11 placed on the steering column 3, and thus measuring the torque exerted by the driver on the steering wheel 2, and an electric signal coming from a speed sensor 12 of the vehicle.
From these various pieces of information, the electronic computer 10 controls the electric assist motor 8, by defining a torque or assist force at any moment, able to amplify or, on the contrary, offset the force applied by the driver on the wheel 2, according to predefined “assist laws.”
It must be noted that the torque sensor used in such a power assisted steering system is complex to implement and calibrate.
In order to resolve these drawbacks, it is known, in particular from patent document U.S. Pat. No. 6,250,419, to produce a power assisted steering system without torque sensor.
However, such a power assisted steering system is likely to cause instabilities in the control of the assist motor that can compromise the safety of the driver and passengers in an automobile equipped with such a power assisted steering system.
Indeed, under limit usage conditions (e.g. under conditions with very low traction, very sudden changes of traction of the black ice patch type), the automobile's driver's feeling, i.e. the torque the driver must exert on the steering wheel to move it, can be inconsistent with the driving situation and therefore bother the driver. This disruption can cause undesired movements of the steering wheel, and thereby compromise the safety of the driver and any passengers in the automobile.